1. Field of the Invention
The invention relates generally to the field of commercial refrigeration, and in a more particular sense, has reference to defrosting of an upwardly opening, low-bed, refrigerated food display case by the use of ambient air.
It may be appropriately considered, accordingly, that the field of the invention has to do with display cases in which the food product is accessible through a continuously open access space, over which an air curtain is maintained at least during the refrigeration mode. It may be further considered that the field of the invention relates to defrosting of such cases, by use of a fluid (air) drawn from outside the refrigerating system itself, that is, the ambient atmosphere surrounding the case.
2. Description of the Prior Art
Defrosting of refrigerated display cases of the type described above has been accomplished, usually, by "hot gas" (or sometimes "cool gas") directed through the tubing of an evaporator; or by electrical heating elements. Most recently, the defrosting of the evaporators by means of air drawn from the surrounding ambient has also received attention as a commercially practicable method of defrosting cases of this type. It has been contended, on behalf of air defrost, that a hot gas defrost system complicates and increases the amount of piping to an undesirable extent; and that electrical defrost consumes too much energy.
These contentions may or may not be valid. However, it is obviously desirable in any event to offer additional defrost methods if at all possible, to suit the needs or desires of particular customers. Accordingly, within the last twenty years, considerable effort has been exerted in the industry to develop workable air defrost systems.
Probably the earliest U.S patent relating to air defrost of a refrigerted display case of the air curtain type is Beckwith U.S. Pat. No. 3,082,612 issued in 1963. This related to a case having multiple air curtains. Most of the patents issued since that data, covering air defrost for display cases of the air curtain type, have similarly related to cases having multiple air curtains, typically found in open front, upright cases.
Low-bed, upwardly opening cases of the air curtain type are usually single curtain cases. There are problems in developing air defrost for this type of case, that are peculiar thereto, and various approaches have been made to effect a workable air defrost for these low-bed, single curtain display cases.
An early disclosure of air defrost for low-bed, single curtain cases, possibly the earliest, is the U.S. patent to Spencer, U.S. Pat. No. 3,226,945. In this patent, the suggestion is for use of pivoted dampers or baffles to change the air pattern. A German publication Offenlegungsschrift No. 2123646 employs ambient air in reverse flow defrost to lower the required electrical input to a defrost heater element.
U.S. Pat. No. 4,120,174 issued to Johnston in 1978 is also of the type in which the air flow is reversed to change the air pattern. In this patent, air defrost is accomplished by the mentioned reversal of the direction of air movement, together with the use of a specially designed bi-oriented air return grille, and/or a means for increasing or allowing unrestrained air flow during defrost as compared to that occurring during normal refrigeration.
The most recent U.S. patent covering air defrost in low-bed cases is believed to be Ljung U.S. Pat. No. 4,182,130. In this patent, which relates specifically to so-called island cases having back-to-back display wells, a mechanical means is utilized to physically relocate the air return, to change the air pattern during defrost as compared to refrigeration.
All of these developments, while very possibly being entirely feasible from a commercial standpoint, are thought to have specific deficiencies or inadequacies that should, if possible, be eliminated. For example, it is desirable to avoid pivoted dampers as much as possible, since they tend to malfunction over a period of time and are difficult to maintain at full efficiency. Simple reversal of air flow may also be undesirable in this particular kind of case, for the reason that it may completely destroy an air curtain during defrost. In some cases, it may be desirable to maintain a weak air curtain, to protect the displayed products, during the defrost mode, especially in view of the fact that it is not practical to provide refrigeration for the products in a single curtain case, during the defrost mode. Indeed, in some instances it may be impossible to do so.
It is also desirable to achieve a case design in which a carefully conceived pre-selection can be made of the air volume removed from the air curtain and exhausted to atmosphere so as to in turn pre-select both the volumetric content, if any, of the air curtain during defrost, and the resultant volume of air drawn into the air passage from the surrounding ambient through the air return. It is obviously desirable to impart these characteristics to the equipment without incorporating therein pivoted dampers, venturis, or the like, and it is also desirable to design the case in such a manner as to incorporate adjustability into the defrost air system, again without the use of pivoted dampers or other mechanical expedients.
Finally, it may be observed that air-defrosting of low-bed cases through simple reversal of the circulating fans, has been found to be satisfactory in meat cases, but results in over-long and hence unacceptable defrost cycles when incorporated in low-bed freezers. The present invention is particularly designed, accordingly, for efficient defrosting of low-bed freezer cases, but may be usable to equal advantage in defrosting meat cases should the use of reversible fans be considered undesirable in that application.